Contact lenses are roughly divided into soft contact lenses and hard lenses. Soft contact lenses, in particular water-containing soft contact lenses, are widely popular because they are less foreign body sensation and provide a comfortable feeling, although the feeling of wearing the lenses fluctuates to a large extent and exerts various influences to the wearer as the surface structure of contact lens changes. The feeling of wearing a contact lens is significantly influenced by the interaction of the contact lens and tear fluid. Therefore, it is important for the lens surface to be covered by tear fluid in order to achieve a comfortable feeling of wearing contact lens. However, when a water-containing soft contact lens falls short of moisture due to evaporation while it is being worn, the soft contact lens covers the moisture shortage by absorbing moisture from tear fluid, resulting in reducing the thickness of the tear fluid layer of the wearer's eye. Then, as a result, the drying is accelerated to damage the feeling of wearing the contact lens and, what is worse, give rise to a risk of causing an outbreak of disorder at the surface of the keratoconjunctive of the contact lens wearing eye due to mechanical friction. Additionally, in recent years, drying of contact lenses and eye surfaces due to external factors including dry eye syndrome, in particular evaporative dry eye, caused by popularization of OA equipment including personal computers and influence of air conditioning systems has been apprehended. A dried contact lens can give rise to adhesion of body wastes in tear fluid to the surface of the contact lens, which causes an uncomfortable feeling of wearing the contact lens and a poor visibility. Therefore, it is vitally important to suppress drying of the contact lens and stabilize the tear fluid layer of a contact lens wearing eye in order to maintain a comfortable feeling of wearing the contact lens. For this purpose, various techniques have been disclosed, which include techniques of improving the anti-contamination property, the hydrophilicity and the wettability of a contact lens to reduce the contamination and drying of the contact lens surface by improving the base material itself of the contact lens and subjecting the obtained contact lens to a hydrophilic treatment.
For example, a water-containing contact lens that reduces the accumulation of body wastes in tear fluid on the surface of the contact lens due to the electric charge of the body wastes by forming zwitter ions including quaternary ammonium ions and carboxyl ions has been disclosed (Referenced Patent Publication 1). While this provides a technique of improving the anti-contamination property of a contact lens caused by the structure of the base material thereof, further efforts are required to improve the property of maintaining the wettability of the base material of the contact lens.
Besides, methods of improving the wettability of a contact lens by adding a moisturizing agent to a prepared contact lens have been disclosed, including a method of forming a hydrated layer on the surface of a contact lens with carboxymethyl cellulose that is an anionic polymer to impart effective lubricating ability (Referenced Patent Publication 2), a method of significantly improving the comfortableness and the moisturizing characteristic of a lens surface by causing a certain poly(oxyethylene)-poly(oxypropylene) surfactant to be retained on the lens surface (Referenced Patent Publication 3) and a method of manufacturing an ophthalmic lens by treating the ophthalmic lens with a moisturizing agent.
However, the method of Referenced Patent Publication 2 coats a contact lens surface with an anionic polymer to provide the lens surface with a wettability but the polymer does not establish any chemical bond with the lens base material so that the polymer is poorly retained on the surface. Additionally, the method merely moisturizes the lens surface and hence cannot improve the moisturizing effect of the entire contact lens. Furthermore, the lens surface is electrically charged with anions, which can induce precipitation of proteins. With the method of Referenced Patent Publication 3, the bond between the hydrophilic film and the contact lens surface is a fragile bond produced by a hydrophobic interaction and hence the method is less reliable in terms of stability and durability of the wettability. Additionally, there are reports that the non-ionic surfactant that the invention employs can induce an anaphylactic shock when administered excessively. Thus, the method is accompanied by a problem of safety that needs to be dissolved. The method of REFERENCED PATENT PUBLICATION 4 causes a contact lens to move to a large extent on the eye surface under the effect of polyvinylpyrrolidone that is employed as moisturizing agent so that a foreign body sensation can easily arise. Additionally, mucin contained in tear fluid can adhere to the contact lens surface due to the stickiness of polyvinylpyrrolidone so that a poor visibility is anticipated when this method is employed.